1. Field of the Invention
The present invention relates to a CMOS compatible pixel cell and, more particularly, to a CMOS compatible pixel cell that utilizes a gated diode to reset the cell.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based signal processors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 shows an example of a conventional CMOS active pixel sensor cell 100. As shown in FIG. 1, cell 100 includes a photodiode 112, a reset transistor 114, whose source is connected to photodiode 112, a sense transistor 116, whose gate is connected to photodiode 112, and a select transistor 118, whose drain is connected in series to the source of sense transistor 116.
Operation of active pixel sensor cell 100 is performed in three steps: a reset step, where cell 100 is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
As shown in FIG. 1, during the reset step, the gate of reset transistor 114 is briefly pulsed with a reset voltage that turns on transistor 114 which, in turn, resets photodiode 112 to an initial integration voltage which is approximately equal to the voltage on the drain of transistor 114 less the threshold voltage of transistor 114.
During integration, light energy, in the form of photons, strikes photodiode 112, thereby creating a number of electron-hole pairs. Photodiode 112 is designed to limit recombination between the newly formed electron-hole pairs. As a result, the photogenerated holes are attracted to the ground terminal of photodiode 112, while the photogenerated electrons are attracted to the positive terminal of photodiode 112 where each additional electron reduces the voltage on photodiode 112.
Thus, at the end of the integration period, the number of photons which were absorbed by photodiode 112 during the image integration period can be determined by subtracting the voltage at the end of the integration period from the voltage at the beginning of the integration period.
Following the image integration period, active pixel sensor cell 100 is read out by turning on select transistor 118. At this point, the reduced voltage on photodiode 112, less the threshold voltage of sense transistor 116, is present on the source of sense transistor 116.
When select transistor 118 is turned on, the voltage on the source of sense transistor 116 is then transferred to the source of select transistor 118. The reduced voltage on the source of select transistor 118 is detected by conventional detection circuitry which includes analog-to-digital (A/D) converters.
Thus, active pixel sensor cell 100 provides a CMOS compatible alternative to CCD-based imagers. However, although active pixel sensor cell 100 is CMOS compatible, there is a need for other pixel cells which are CMOS compatible.
The present invention provides a pixel cell that, in addition to being CMOS compatible, utilizes a gated diode to reset the cell. By utilizing a gated diode to reset the cell, the present invention eliminates the need for A/D converters which are conventionally used in pixel cells.
A pixel cell in accordance with the present invention, which is formed in a semiconductor material of a first conductivity type, includes a first well of a second conductivity type which is formed in the semiconductor material, and a second well of the first conductivity type which is formed in the first well.
In addition, the pixel cell also includes a gated diode which is formed in the second well, a read out transistor which is formed in the second well, and a cell diode which is connected between the gated diode and ground. Further, the read out transistor is spaced apart from the gated diode.
The present invention also includes an imaging system, which is formed in a semiconductor material of a first conductivity type, that includes a pixel cell and a control circuit. The pixel cell includes a first well of a second conductivity type which is formed in the semiconductor material, and a second well of the first conductivity type which is formed in the first well.
The pixel cell also includes a gated diode which is formed in the second well, a read out transistor which is formed in the second well, and a cell diode which connected between the gated diode and ground. In addition, the read out transistor is spaced apart from the gated diode.
The control circuit includes an oscillator which is connected to the gated diode, a counter which is connected to the gated diode, and a controller which is connected to the oscillator, the gated diode, and the read out transistor.
The present invention further includes an imaging system, which is formed in a semiconductor material of a first conductivity type, that includes a plurality of pixel cells which are formed in rows and columns, and a plurality of row select lines which are connected to the pixel cells so that each row select line is connected to the pixels in a row of pixels.
The imaging system also includes a plurality of oscillator output lines which are connected to the pixel cells so that each oscillator output line is connected to the pixels in a row of pixels, and a plurality of read out lines which are connected to the pixel cells so that each read out line is connected to the pixels in a column of pixels.
The imaging system additionally includes a plurality of counters which are arranged so that a counter corresponds with each column of pixel cells, and a counter output line which is connected to each counter. Further, a plurality of control lines are connected to the counters so that each control line is connected to a counter, and an oscillator/controller is connected to the row select lines, the oscillator output lines, the read out lines, the counter output line, and the control lines.
The pixel cell of the present invention is operated by setting a potential on the second well to a first level. A plurality of photons are then collected in the first and second wells. The plurality of photons changes the potential on the second well to a second level.
A number of pulses are next applied to the gated diode until the potential on the second well returns to the first level. The number of pulses applied to the gated diode that are required to return the potential on the second well to the first level are counted to determine the number of absorbed photons.
The second imaging system of the present invention is operated by selecting via the controller a row of pixel cells to be read out, and then applying a row select voltage to the row select line that corresponds with the row of pixel cells to be read out.
A number of pulses are next applied to the oscillator output line that corresponds with the row of cells to be read out. The pulses change a potential on each of the cells to be read out. The number of pulses required to set the potential of each cell to a first level is counted to determine the number of photons absorbed by each cell.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized.